Corporation Tax Deadlines for Limited Companies

A limited company has several Corporation Tax deadlines that do not fall on the same day. You must register within three months of starting to trade, pay your Corporation Tax nine months and one day after your accounting period ends, and file your Company Tax Return (the CT600) twelve months after that same period. Oddly, you pay before you file. Separate annual accounts also go to Companies House within nine months. Miss any of these and automatic penalties and interest follow.

Corporation Tax has a rhythm of its own, and it catches out plenty of new directors. The dates are driven by your company’s accounting period, which usually matches your financial year. Once you know your period end, every other deadline follows from it. Below we set out exactly what is due and when, and why the payment date and the filing date are deliberately different.

When must a new company register for Corporation Tax?

You must tell HMRC that your company is active and register for Corporation Tax within three months of starting to do business. Starting to do business usually means trading, but it also covers buying, selling, advertising, renting a property or employing someone. Registering late can lead to a penalty, so it is best to do it as soon as the company begins operating. If your company is dormant and doing nothing at all, different rules apply and you should tell HMRC it is dormant instead. For a fuller picture of how the tax itself works, see our guide to Corporation Tax explained.

When do you have to pay Corporation Tax?

For most companies, those with taxable profits up to £1.5 million, the payment deadline is nine months and one day after the end of your accounting period. So if your period ends on 31 March, your Corporation Tax is due by 1 January the following year. There is no bill or reminder that arrives on your doormat telling you the exact figure, so you or your accountant need to work it out and pay it on time. Interest runs from the day after the deadline on anything paid late, and that interest is not something HMRC waives lightly.

When is the Company Tax Return due?

Your Company Tax Return, filed on form CT600, is due twelve months after the end of your accounting period. Using the same 31 March example, the return deadline is the following 31 March, a full three months after the tax itself was payable. The return reports your profit, your tax calculation and any reliefs claimed, and it is filed online together with your accounts and computations. First returns can be slightly more involved, because a company’s first accounts often cover a period longer than twelve months and may need to be split into two returns.

Why are the payment and filing dates different?

This is the point that trips people up most. You pay your Corporation Tax at nine months and one day, but you do not have to file the return until twelve months. In other words, you pay before you formally file. In practice most accountants prepare the accounts and the CT600 first, arrive at the figure, then pay by the earlier deadline and submit the return afterwards. Leaving the calculation until the twelve month filing date means you will already have missed the payment date and started clocking up interest.

What about companies with very large profits?

Companies with taxable profits over £1.5 million do not get the nine months and one day rule. Instead they must pay their Corporation Tax in quarterly instalments, with the first instalment falling due during the accounting period itself rather than after it. Companies with profits over £20 million, treated as very large, pay on an even earlier instalment schedule. If your profits sit near the boundary, or you are working through the thresholds that affect your rate, our guide to marginal relief on Corporation Tax is worth a read.

Do you still have to file accounts with Companies House?

Yes, and this is a completely separate obligation. Your annual accounts must reach Companies House within nine months of your accounting reference date (twenty one months after incorporation for a brand new company’s first accounts). This sits alongside your annual confirmation statement, which keeps your company details up to date. Filing with Companies House does not satisfy HMRC, and filing the CT600 does not satisfy Companies House. Two filings, two deadlines, two different bodies.

What are the penalties for filing late?

The late filing penalties for the CT600 escalate quickly. You pay £200 as soon as the return is one day late, another £200 once it is three months late, then a penalty of 10 per cent of any unpaid tax at six months, and a further 10 per cent at twelve months. If your return is late three times in a row, the £200 penalties rise to £1,000 each. Late payment of the tax is handled separately through interest. You can browse the wider timetable in our accountancy and tax guide, and the parallel rules for individuals in Self Assessment deadlines and penalties.

HMRC enquiries into company tax returns are common, and they can arrive even when everything has been filed correctly and on time. A Solar Protect tax investigation insurance policy covers up to £100,000 of your accountant’s professional fees in defending you, with no excess to pay. To protect your business, get a quote today.

Reviewed by Matthew Bartlett, Director, Solar Protect. This guide is general information, not personalised tax or financial advice. Figures are correct for the 2025/26 and 2026/27 UK tax years at the time of writing.